The present invention relates to a producing method of a fatty acid silver salt for use in a photothermographic image-recording material, preferably a photothermographic material. The present invention also relates to a photothermographic image-recording material using the fatty acid silver salt which is used as an excellent photothermographic image-recording material for medical diagnosis, whose image-forming layer at the cut end hardly peels off when cut to an H cut (35 cmxc3x9743 cm), B4 size or the like.
A variety of photographic materials comprising a support having provided thereon a photosensitive layer and forming an image by image exposure are known.
Reduction of waste solution has been strongly desired in recent years in the field of photomechanical process and in the medical field from the viewpoint of environmental protection and space saving. Accordingly, a technique concerning a photothermographic image-recording material for photomechanical process and medical use which can be exposed efficiently with laser beams, from which a clear black image having high resolving power and sharpness can be formed has been required. Such a photothermographic image-recording material can offer to customers a simpler and environmentally benign heat development processing system in which the use of solvent system processing chemicals can be done away with.
An image-forming method by heat development is described, e.g., in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Klostervoer, Thermally Processed Silver Systems, xe2x80x9cImaging Processes and Materialsxe2x80x9d, compiled by Sturge, V. Walworth and A. Shepp, Noblette 8th Ed., Chap. 9, p. 279 (1989). The photothermographic image-recording material contains in general a reducible photo-insensitive silver source (e.g., an organic silver salt), a catalytically active amount of photocatalyst (e.g., a silver halide), and a reducing agent of silver dispersed in an organic binder matrix. A photothermographic image-recording material is stable at normal temperature but forms a silver by heating at high temperature (e.g., 80xc2x0 C. or more) after image exposure through an oxidation reduction reaction between the reducible silver source (which functions as an oxidizing agent) and the reducing agent. The oxidation reduction reaction is accelerated by the catalytic action of the latent image generated by exposure. The silver formed by the reaction of the reducible silver source in the exposed domain offers a black image contrasting with the non-exposed domain to thereby form an image.
The silver sources which are used in these systems are in general a fatty acid silver salt, and various producing methods are known. For example, a method of producing an organic silver salt in the coexistence of water and a hardly water-soluble solvent as disclosed in JP-A-49-93310 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-49-94619, and JP-A-53-68702, a method of producing an organic silver salt in an aqueous solution as disclosed in JP-A-53-31611, JP-A-54-4117, and JP-A-54-46709, and a method of producing an organic silver salt in an organic solvent as disclosed in JP-A-57-186745, JP-A-47-9432 and U.S. Pat. No. 3,700,458. A fatty acid silver salt is fundamentally produced by heating a fatty acid in water to higher than the melting point, adding a sodium hydroxide or an alkali metal salt to the molten fatty acid with vigorously stirring, and then adding a silver nitrate to convert an alkali soap to a silver soap.
Such an alkali soap forms micelle in an aqueous solution and an alkali soap is a white turbid solution in appearance. The reaction from micelle to silver soap often causes a problem of production stability. Therefore, a method of using a mixture of water and alcohol as a solvent is disclosed in JP-A-55-40607 for making an alkali soap a uniform solution.
Further, a method of making an alkali soap a uniform solution by using a mixed solvent of water and alcohol and adding the alkali soap with a silver solution for improving photographic properties is disclosed in JP-A-11-349325.
It has become possible to produce a fatty acid silver salt by a uniform reaction by using the simultaneous addition method disclosed in JP-A-11-349325. However, when a fatty acid alkali metal salt solution and a silver ion solution are added to a reaction tank as disclosed in the example of the above publication, foams are generated during the reaction and the viscosity of the solution increases at the final stage of the reaction, therefore, there is an anxiety in scale up.
Therefore, a method of adding a fatty acid alkali metal salt solution and a silver ion solution into a closed mixing means is discussed. However, the hydrophilic property of the surface of the particle decreases by adding the solutions into a closed mixing means, which leads to another problem of the deterioration of the film-forming property.
The deterioration of the film-forming property of photothermographic image-recording materials has come to a great problem to each manufacturer. If the film-forming property is bad, the inside of the layer containing a fatty acid silver is destroyed by the shock of the work blade when the photothermographic image-recording material is cut to a prescribed size and peeling is caused. As a result, the peeled part falls off during the succeeding transportation and the like and results in the trouble of a blank area. For example, the end face of a commercially available photothermographic image-recording material, Dry View Laser Imaging Film manufactured by Eastman Kodak Co. also suffers film peeling. For producing a photothermographic image-recording material, it is necessary to form a layer by dispersing a fatty acid silver salt in a binder, and the problem is attributed to the fact that the film of a fatty acid silver salt is difficult to form as compared with the film of ordinary silver halide. This is presumably due to the fact that the volume occupied by a fatty acid silver salt per mol is large hence the volume of the binder occupied by a fatty acid silver salt is also large. A method to cope with this problem by increasing the ratio of a binder is known at present, however, this method also influences photographic properties and attended by many disadvantages. Therefore, it has been required to improve film-forming property of a fatty acid silver salt without affecting photographic properties.
In view of the above problems of the conventional techniques, an object of the present invention is to provide a producing method of a fatty acid silver salt which can prevent the occurrence of the trouble of a blank area attributable to film-forming hindrance and can lessen fog when used in a photothermographic image-recording material (in particular, a photothermographic material). Another object of the present invention is to provide a photothermographic image-recording material, in particular a photothermographic material which can control the trouble of a blank area attributable to film-forming hindrance and lessens fog.
As a result of eager investigation, the present inventors have found that a fatty acid silver salt showing the expected effect can be obtained by mixing a silver ion solution and a fatty acid alkali metal salt solution according to the prescribed conditions. Thus, the present invention has been achieved.
That is, the present invention provide a producing method of a fatty acid silver salt which comprises adding (1) a solution of silver ions comprising water, or a mixed solution of an organic solvent and water, containing therein silver ions, and (2) a solution of a fatty acid alkali metal salt which is a solution or a suspension comprising water, an organic solvent, or a mixed solution of water and an organic solvent, containing therein an alkali metal salt of a fatty acid to a closed mixing means to react the solution (1) and the solution (2), wherein from 50 to 99.5 mol % of the entire fatty acid alkali metal salt solution is added to the closed mixing means under such a condition that the concentration of the fatty acid alkali metal salt is higher than the silver ion concentration, and from 0.5 to 30 mol % of the entire fatty acid alkali metal salt solution is added to the closed mixing means or to the downstream of the closed mixing means after the silver ion solution has been added to the closed mixing means. In the producing method of a fatty acid silver salt according to the present invention, it is preferred that from 0.5 to 30 mol % of the entire fatty acid alkali metal salt solution is added to a formation tank equipped downstream from the closed mixing means after the silver ion solution has been added to the closed mixing means.
Further, the present invention provide a photo-thermographic image-recording material comprising a support having provided thereon a reducing agent, a binder and a photo-insensitive organic silver salt, wherein the fatty acid silver salt produced by the above-described producing method is used as the photo-insensitive organic silver salt. It is preferred that the photothermographic image-recording material according to the present invention further contain a photosensitive silver halide on a support, and further, the ratio of the aqueous latex solid content weight to the fatty acid silver weight in the layer containing the fatty acid silver salt be from 1.0 to 2.5.
In the present invention, xe2x80x9cfrom x to yxe2x80x9d means the range including the numerical values x and y as the minimum value and maximum value respectively.